C-fms and c-kit are both type III transmembrane receptor protein tyrosine kinases (RPTKs) that regulate key signal transduction cascades that control cellular growth and proliferation. Both receptors have similar structural features comprising five extracellular immunoglobulin (IG) domains, a single transmembrane domain, and a split cytoplasmic kinase domain separated by a kinase insert segment.
c-Fms
C-fms is a member of the family of genes originally isolated from the Susan McDonough strain of feline sarcoma viruses. The cellular proto-oncogene FMS (c-fms, cellular feline McDonough sarcoma) codes for the receptor for the macrophage colony-stimulating factor (M-CSF). C-fms is crucial for the growth and differentiation of the monocyte-macrophage lineage, and upon binding of M-CSF to the extracellular domain of c-fms, the receptor dimerizes and trans-autophosphorylates cytoplasmic tyrosine residues.
M-CSF, first described by Robinson and co-workers (Blood. 1969, 33:396-9), is a cytokine that controls the production, differentiation, and function of macrophages. M-CSF stimulates differentiation of progenitor cells to mature monocytes, and prolongs the survival of monocytes. Furthermore, M-CSF enhances cytotoxicity, superoxide production, phagocytosis, chemotaxis, and secondary cytokine production of additional factors in monocytes and macrophages. Examples of such additional factors include granulocyte colony stimulating factor (G-CSF), interleukin-6 (IL-6), and interleukin-8 (IL-8). M-CSF stimulates hematopoiesis, promotes differentiation and proliferation of osteoclast progenitor cells, and has profound effects on lipid metabolism. Furthermore, M-CSF is important in pregnancy. Physiologically, large amounts of M-CSF are produced in the placenta, and M-CSF is believed to play an essential role in trophoblast differentiation (Motoyoshi, Int J. Hematol. 1998, 67:109-22). The elevated serum levels of M-CSF in early pregnancy may participate in the immunologic mechanisms responsible for the maintenance of the pregnancy (Flanagan & Lader, Curr Opin Hematol. 1998, 5:181-5).
Related to c-fms and c-kit are two platelet-derived growth factor receptors, alpha (i.e., pdgfra) and beta (pdgfrb) (PDGF). The gene coding for pdgfra is located on chromosome 4q11-q12 in the same region of chromosome 4 as the oncogene coding for c-kit. The genes coding for pdgfra and c-fms appear to have evolved from a common ancestral gene by gene duplication, inasmuch as these two genes are tandemly linked on chromosome 5. They are oriented head-to-tail with the 5-prime exon of the c-fms gene located only 500 bp from the last 3-prime exon of the gene coding for pdgfra. Most gastrointestinal stromal tumors (GIST) have activating mutations in c-kit, and most patients with GISTs respond well to Gleevec, which inhibits c-kit. Heinrich et al. (Science 2003, 299:708-10) have shown that approximately 35% of GISTs lacking c-kit mutations have intragenic activation mutations in the gene encoding pdgfra, and that tumors expressing c-kit or pdgfra are indistinguishable with respect to activation of downstream signaling intermediates and cytogenetic changes associated with tumor progression. Thus, c-kit and pdgfra mutations appear to be alternative and mutually exclusive oncogenic mechanisms in GISTs.
Similarly, the observation that production of M-CSF, the major macrophage growth factor, is increased in tissues during inflammation points out a role for c-fms in diseases, such as for example inflammatory diseases. More particularly, because elevated levels of M-CSF are found in the disease state, modulation of the activity of c-fms can ameliorate disease associated with increased levels of M-CSF.
c-Kit
The Stem Cell Factor (SCF) receptor c-kit plays an important role in the development of melanocytes and mast, germ and hematopoietic cells. Stem Cell Factor (SCF) is a protein encoded by the 51 locus, and has also been called “kit ligand” (KL) and mast cell growth factor (MGF), based on the biological properties used to identify it (reviewed in Tsujimura, Pathol Int 1996, 46:933-938; Loveland, et al., J. Endocrinol 1997, 153:337-344; Vliagoftis, et al., Clin Immunol 1997, 100:435-440; Broudy, Blood 1997, 90:1345-1364; Pignon, Hermatol Cell Ther 1997, 39:114-116; and Lyman, et al., Blood 1998, 91:1101-1134). Herein the abbreviation SCF refers to the physiological ligand for c-kit.
SCF is synthesized as a transmembrane protein with a molecular weight of 220 or 248 Dalton, depending on alternative splicing of the mRNA to encode exon 6. The larger protein can be proteolytically cleaved to form a soluble, glycosylated protein which noncovalently dimerizes. Both the soluble and membrane-bound forms of SCF can bind to and activate c-kit. For example, in the skin, SCF is predominantly expressed by fibroblasts, keratinocytes, and endothelial cells, which modulate the activity of melanocytes and mast cells expressing c-kit. In bone, marrow stromal cells express SCF and regulate hematopoiesis of c-kit expressing stem cells. In the gastrointestinal tract, intestinal epithelial cells express SCF and affect the interstitial cells of Cajal and intraepithelial lymphocytes. In the testis, sertoli cells and granulosa cells express SCF which regulates spermatogenesis by interaction with c-kit on germ cells.